The Curious Case of Benjamin Y-axis Button

I came across a strange behavior of QT5 on raspberry.
I'm using a TFT display with resistive touchscreen on a raspberry pi 2.
The external display is configured as a frame buffer /dev/fb1 while, as usual, monitor is /dev/fb0.
The touchscreen is /dev/input/touchscreen but is a link to /dev/input/event3.
I made a stupid application with a few sliders, button etc but does absolutely nothing except exiting the application when quit is pressed.
There are also two QLCDNumber indicating the x, y coordinates of the pointer.
I start the application with -platform linuxfb: fb=/dev/fb1 -style fusion -plugin tslib
Tslib is calibrated and with environment set correctly.
When I launch, the application is displayed correctly but if I interact via touchpanel you'll immediately notice that something is wrong.
In framebuffer coordinates 0.0 are top, left and in fact the graphics are correct.
Unfortunately QtLcd 2 show that the coordinates 0,0 for the touch panel are bottom, left.
I recalibrated the tslib and tried various tslib tools and is entirely correct, but always in Qt the y-axis still reversed.
After 1 week of attempts I decided to try to use the framebuffer /dev/fb0 to see if the same happens and magically the application works with the y-axis in the right direction using the calibrated touchplanel.

After another week of trial, while looking at the sources in QTBase, I have found a variable QT_QPA_FB_TSLIB and after setting it to 1 the y-axis is back in the right direction using properly tslib and now everything works even using /dev/fb1 (I obviously omitted -plugin tslib from the command line)

I have the impression that there is a bug in Qt5.5 that makes "-plugin tslb" functions only on the framebuffer 0 or that, I do not know why, goes wrong for /dev/fb1.